FIG. 4 shows a flow sheet of a conventional ultra high purity nitrogen and oxygen generator unit described in the official gazette of Japanese Patent Application Laid-open (KOKAI) No. 296,651/1993. In the drawing, the reference numeral 54 represents a first rectification column, 55 represents a second rectification column, 56 represents a third rectification column, 57 represents a fourth rectification column, 58 represents a nitrogen condenser, 53 represents a main heat exchanger and 59 represents an expansion turbine, respectively.
After feed air is compressed, it is freed of carbon dioxide and moisture, and then cooled down by the main heat exchanger 53, whereby a portion of the feed air is introduced into a lower space part 54e of the first rectification column 54 as it is liquefied. The liquid phase portion of the feed air introduced in the lower space part 54e collects in the bottom of the lower space part 54e and the gas phase portion thereof is caused to rise through the first rectification column 54, i.e. to pass in turn through a lower rectifying part 54d, a middle rectifying part 54c and an upper rectifying part 54b so as to be brought in countercurrent contact with a reflux liquid consisting mainly of liquid nitrogen, which flows down from above. Accordingly, oxygen and mainly components (hydrocarbons, krypton, xenon, etc.) having higher boiling points than that of oxygen in the gas phase are absorbed into the reflux liquid, while nitrogen and mainly components (neon, hydrogen, helium, etc.) having lower boiling points than that of nitrogen in the reflux liquid are evaporated and released into the gas phase. As a result, high purity nitrogen gas containing lower boiling point components collects in the upper space part 54a and oxygen-rich liquid air containing higher boiling point components collects in the lower space part 54e.
The high purity nitrogen gas collected in the upper space part 54a is introduced into the nitrogen condenser 58 so as to be cooled down, and the thus-condensed high purity liquid nitrogen is supplied to the upper rectifying part 54b as a reflux liquid again, while non-condensed gas in which the lower boiling point components have been concentrated is discharged out of the system.
A portion of the oxygen-rich liquid air collected in the lower space part 54e is introduced into an expansion valve 61, where it is reduced in pressure so as to get oxygen-rich waste gas having a low temperature, and this oxygen-rich waste gas will be introduced into the nitrogen condenser 58 as a refrigerant. The oxygen-rich waste gas discharged from the nitrogen condenser 58 is further introduced into the expansion turbine 59, used in the main heat exchanger 53 as a refrigerant, and then discharged out of the system.
Liquid nitrogen condensed in the nitrogen condenser 58 and supplied to the upper rectifying part 54b is brought in countercurrent contact with a rising gas mainly consisting of nitrogen as it is flowing down in the upper rectifying part 54b, so as to get ultra high purity liquid nitrogen because the lower boiling point components remaining therein are further released. This ultra high purity liquid nitrogen collects in a reservoir part 54g provided between the upper rectifying part 54b and the middle rectifying part 54c. A portion thereof is extracted out as the ultra high purity liquid nitrogen, reduced in pressure by an expansion valve 63, brought in heat exchange and then supplied to the outside of the system as an ultra high purity nitrogen gas product, and the remaining portion is further caused to flow down through the middle rectifying part 54c as a reflux liquid.
Another portion of the oxygen-rich liquid air collected in the lower space part 54e is fed to an expansion valve 62, where it is reduced in pressure and partially evaporated so as to get a gas-liquid mixture, and this gas-liquid mixture is supplied to above the rectifying part 55b of the second rectification column 55. The gas phase portion of this gas-liquid mixture collects in the upper space part 55a, and the liquid phase portion thereof is caused to flow down through the rectifying part 55b as a reflux liquid, where it is brought in countercurrent contact with a gas rising from below so as to be enhanced in oxygen concentration, with releasing the lower boiling point components, and collects in the lower space part 55c. In the lower space part 55c is installed a reboiler 71 for heating liquid collected in the lower space part 55c so that components (argon, carbon monoxide, nitrogen, etc.) having lower boiling points than that of oxygen are selectively evaporated together with oxygen, and caused to rise through the rectifying part 55b. As a result, liquid oxygen containing higher boiling point components collects in the lower space part 55c and gas containing oxygen, nitrogen and lower boiling point components collects in the upper space part 55a, and they will be discharged out of the system from the column bottom part and column top part, respectively.
Oxygen gas collected in the gas phase portion above the liquid level of the lower space part 55c of the second rectification column 55 is supplied to the lower space part 56c of the third rectification column 56. The oxygen gas supplied therein is brought in countercurrent contact with a reflux liquid (high purity liquid oxygen) as it is rising through the rectifying part 56b, whereby higher boiling point components are absorbed in the reflux liquid and at the same time, a portion of oxygen in the reflux liquid is evaporated. In the upper space part 56a of the third rectification column 56 is installed a condenser 81 for cooling down and condensing gas (high purity oxygen) collected in the upper space part 56a and supplying the thus-condensed gas to the rectifying part 56b as said reflux liquid. And as a result, liquid oxygen containing a trace of higher boiling point components collects in the lower space part 56c and higher purity oxygen gas containing a trace of lower boiling point components collects in the upper space part 56a. The liquid oxygen containing higher boiling point components collected in the lower space part 56c is returned to the lower space part 55c of the second rectification column 55.
High purity-oxygen gas collected in the upper space part 56a is supplied to the middle part 57c between the upper rectifying part 57b and lower rectifying part 57d of the fourth rectification column 57. The high purity oxygen gas supplied therein is brought in countercurrent contact with a reflux liquid (high purity liquid oxygen) as it is rising through the upper rectifying part 57b, whereby oxygen is absorbed in the reflux liquid and at the same time, lower boiling point components in the reflux liquid are evaporated. In the upper space part 57a of the fourth rectification column 57 is installed a condenser 82 for cooling down and condensing gas (high purity oxygen) collected in the upper space part 57a and supplying the thus-condensed gas to the rectifying part 57b as said reflux liquid. In the lower space part 57e, on the other hand, a reboiler 72 is installed which serves to heat liquid (ultra high purity liquid oxygen) collected in the lower space part 57e so that components having lower boiling points than that of oxygen are selectively evaporated together with oxygen and the thus-evaporated components are caused to rise in turn through the lower rectifying part 57d and upper rectifying part 57b so as to be brought in countercurrent contact with the reflux liquid (high purity liquid oxygen). And as a result, ultra high purity liquid oxygen collects in the lower space part 57e and oxygen gas in which the lower boiling point components have been concentrated collects in the upper space part 57a. The oxygen gas collected in the upper space part 57a will be discharged out of the system from the column top part, and the ultra high purity liquid oxygen collected in the lower space part 57e will be recovered as a product and supplied to the outside of the system.
The official gazette of Japanese Patent Application Laid-open (KOKAI) No. 105,088/1988 describes a method of producing nitrogen gas (99.97%) and ultra high purity oxygen gas (99.998%) by use of two rectification columns. According to this method, feed air is fed to the bottom wart of a first rectification column and oxygen-enriched liquid air extracted from a position which is above one equilibrium stage from the lower end of the rectifying part of the first rectification column is fed to the top part of a second rectification column, wherein nitrogen-enriched gas is recovered from the vicinity of the top part of the first rectification column and ultra high purity oxygen gas is recovered from a position which is above one equilibrium stage from the lower end of the rectifying part of the second rectification column (see: FIG. 2 of the official gazette).
Although the unit described in the official gazette of Japanese Patent Application Laid-open (KOKAI) No. 296,651/1993 possesses an advantage that nitrogen of ultra high purity and oxygen of ultra high purity can be produced from one unit only by the liquefaction and rectification of feed air, there are such defects that four rectification columns are required, a piping system is complicated and the operation condition is complicated because of plural condensers and reboilers installed. The method described in the official gazette of Japanese Patent Application Laid-open (KOKAI) No. 105,088/1986 is not one of obtaining ultra high purity nitrogen at the same time.